Skip fire engine control is understood to offer a number of benefits including the potential of increased fuel efficiency. In general, skip fire engine control contemplates selectively skipping the firing of certain cylinders during selected firing opportunities. Thus, for example, a particular cylinder may be fired during one firing opportunity and then may be skipped during the next firing opportunity and then selectively skipped or fired during the next. This is contrasted with conventional variable displacement engine operation in which a fixed set of the cylinders are deactivated during certain low-load operating conditions.
In this manner, even finer control of the effective engine displacement is possible. For example, firing every third cylinder in a 4 cylinder engine would provide an effective displacement of ⅓rd of the full engine displacement, which is a fractional displacement that is not obtainable by simply deactivating a set of cylinders. Similarly, firing every other cylinder in a 3 cylinder engine would provide an effective displacement of ½, which is a fractional displacement that is not obtainable by simply deactivating a set of cylinders. U.S. Pat. No. 8,131,445 (which was filed by the assignee of the present application and is incorporated herein by reference in its entirety for all purposes) teaches a variety of skip fire engine control implementations.
When a cylinder is deactivated in a variable displacement engine, its valves are not actuated and although the piston typically still reciprocates, fuel is not combusted during the power stroke. Since the cylinders that are “shut down” don't deliver any net positive torque, the proportionate load on the remaining cylinders is increased, thereby allowing the remaining cylinders to operate at an improved thermodynamic efficiency. With skip fire control, cylinders are also preferably deactivated during skipped working cycles in the sense that air is not pumped through the cylinder and no fuel is delivered and/or combusted during skipped working cycles when such valve deactivation mechanism is available. Often, no air is introduced to the deactivated cylinders during the skipped working cycles thereby reducing pumping losses. However, in other circumstances it may be desirable to trap exhaust gases within a deactivated cylinder, or to introduce, but not release air from a deactivated cylinder during selected skipped working cycles. In such circumstances, the skipped cylinder may effectively act as a gas spring. Although deactivating skipped cylinders is generally preferred, it should be appreciated that in some engines or during some working cycles it may not be possible, or in some situations desirable, to truly deactivate cylinders. When a cylinder is skipped, but not deactivated, intake gases drawn from the intake manifold are effectively pumped through the cylinder during the skipped working cycle.
Although the concept of skip fire control has been around for a long time, it has not traditionally been used in commercially available engines, so an additional challenge to implementing skip fire control is insuring that the engine's other engine/power train systems work effectively during skip fire control. One such system relates to engine diagnostics. As is well understood by those familiar with the art, modern vehicles incorporate engine management systems that perform in-situ diagnostics on various powertrain and vehicle component during vehicle operation. These diagnostic systems are often referred to as “On-Board Diagnostics” (OBD) systems and there are a number of engine diagnostic protocols that are performed while the engine is running. Modern OBD systems store and report a significant amount of information concerning the operation and state of health of various vehicle sub-systems including the powertrain. For example, some OBD systems are arranged to detect a situation in which a cylinder misfires i.e., when the cylinder fails to fire or there is incomplete combustion in the cylinder.
Although prior art OBD systems are well suited to detect misfire in a conventional all-cylinder engine control system, they are generally ill suited for use in a skip fire engine control system. Various embodiments of the present invention contemplate arrangements, methods and techniques for detecting misfire in an engine operated in a skip fire manner